Compressive strength and modulus of elasticity are recognized as important measurements for quality control in masonry. The non-destructive aspect of NDTs allows for testing of more samples more frequently because the sample is not sacrificed. Moreover, standard destructive test methods are time consuming and require large expensive testing apparatus that is not ubiquitously available. In some places this has led to a disturbing lack of testing and construction of dangerously weak buildings. In this circumstance, development and adoption of reliable, inexpensive NDTs can help to protect public safety and prevent much damage and mortality in the event of an earthquake. Even in countries where testing is better, the availability of reliable inexpensive NDTs can complement and improve existing testing programs.
A number of NDTs have been described in the literature such as M. Forde, “ACI 228.2R-13 Report on Nondestructive Test Methods for Evaluation of Concrete in Structures,” American Concrete Institute, 2013. However, there appears to be no mention of use of sonic signals for measurement of concrete strength. Qualitative assessment in the course of inspection of a specimen may involve tapping the specimen, but such assessment is directed primarily towards detection of localized structural inconsistencies, and no systematic analysis of the “sound” of the tap is applied. There is recognized methodology for systematic analysis of the response to a tap but again this is directed towards characterization of structural inconsistencies. Most of these methods rely on accelerometers that are affixed to the object being tested. There is a description that considers the possibility of picking up the airborne sonic signal using a microphone for assessment of localized structural inconsistencies in I. Hertlin and D. Schultze, “Acoustic Resonance Testing: the upcoming volume-oriented NDT method,” in International Conference for non-destructive testing, Rio de Janeiro, Brazil, 2003, but no mention is made of strength measurement in this paper.
A method that shares some similarities with our methodology is found in ASTM, “C215-02: Standard Test Method for Fundamental, Longitudinal and Torsional Resonance Frequencies of Concrete Specimens,” ASTM International, 2002, wherein a method is described for measuring the transverse, longitudinal and torsional resonant frequencies of concrete specimens. One variant of this method involves striking the specimen with a hammer and analysing a signal that is picked up by an accelerometer attached to the sample. The data can be used to estimate the dynamic modulus of elasticity, dynamic modulus of rigidity and dynamic Poisson's ratio from the resonant frequencies, mass and dimensions of the sample. However, no mention is made of measuring the compressive strength of the sample, and no mention is made of using an airborne sonic signal in the analysis.
There are several patents related to ultrasonic testing, such as:
U.S. Pat. No. 7,587,943, H. Wiggenhauser and A. Samokrutov, “Device for the destruction-free testing of components”;
U.S. Pat. No. 7,587,943, 15 Sep. 2009;
U.S. Pat. No. 7,677,104 B2, V. E. Maki and J. J. Moon Jr, “Acoustic Transducer System for Non-Destructive Testing of Cement”;
U.S. Pat. No. 7,677,104 B2, 16 Mar. 2010;
UK1279865 J. Vainshtok, N. Mizrokhi and I. I. Silkin, “Improvements in and relating to Ultrasonic Testing Apparatus”;
United Kingdom Patent 1,279,865, 28 Jun. 1972, UK1262343, A Transducer for the testing of materials by use of ultra-sonic vibrations”;
United Kingdom Patent 1,262,343, 2 Feb. 1972;
DE19629485, R. Krompholz, B. Kaesner, J. Oecknick and G. Gebauer, “Ultrasonic measurement of concretre compressive strength to determine time for demoulding setting concrete”; and
Europe Patent 19,629,485, 22 Jan. 1998.
The methods, apparatus and objectives of these above noted patents differ significantly from ours.
Patents related to strength testing of concrete slurry or curing concrete, such as: (i) U.S. Pat. No. 6,112,599, V. Maki Jr, “Method and Apparatus for Measuring a Cement Sample using a Single Transducer Assembly”; (ii) U.S. Pat. No. 6,112,599, 5 Sep. 2000; (iii) U.S. Pat. No. 6,510,743, R. G. McAfee and R. E. Carpenter Jr, “Reuseable in-situ concrete Test specimen apparatus and method”; (iv) U.S. Pat. No. 6,510,743, 28 Jan. 2003; (v) US2007/0046479 A1, J. H. Hines, “Concrete Maturity Monitoring System using Passive Wireless Surface Acoustic Wave Temperature Sensors”; and (vi) United States Patent 2007/0046479 A1, 1 Mar. 2007, are also quite different from ours, as are methods for detecting imbedded defects in concrete structures, such as (i) U.S. Pat. No. 4,896,303, D. Leslie, J. A. E. De Selliers de Moranville and D. J. Pittman, “Method for Cementation Evaluation using Acoustical Coupling and Attenuation”; (ii) U.S. Pat. No. 4,896,303, 23 Jan. 1990 (iii) U.S. Pat. No. 4,748,855, R. M. Barnoff, “Device for in-situ testing of concrete”; and (vi) U.S. Pat. No. 4,748,855, 7 Jun. 1988.
U.S. Pat. No. 4,342,229 describes an invention for measuring degradation of the structural integrity of objects that may be susceptible to small structural defects or fatigue failures. While an acoustic impulse response method is described and signals may be recorded using either accelerometers or microphones, their method relies on comparing the rate of decay of the impulse response with reference values for such decay. This analysis is significantly different from the present invention, and no mention is made of measuring compressive strength.
U.S. Pat. No. 5,285,687 describes and invention that employs acoustic testing for damage of monolithic carrier elements of porous ceramic material for use in manufacturing of waste gas catalysts. While the methods involve acoustic impulse responses, the application, the apparatus and the analysis methods are fundamentally different and compressive strength is not mentioned.
Finally, U.S. Pat. No. 6,990,845, W. S. Voon, R. U. Spjado, A. M. S. Hamouda, M. M. H. Ahmad and T. K. Sheng, “Pendum Impact Test Rig”, Jan. 31, 2006 describes a pendulum-based tester where the impulse response of the sample under test is analysed. However, this is primarily described for simulation of crash conditions on individual automotive components. The component under test is typically damaged or destroyed in the envisaged application, and no mention is made of non-destructive testing of concrete.
There appears to be no patents directed at non-destructive measurement of compressive strength of concrete samples. The systematic use of airborne sonic signal for this type of testing also appears to be unexplored. Even with methods that use accelerometers the connection to compressive strength has not been identified.